The User Equipments (UEs) of the Wideband Code Division Multiple Access (WCDMA) uplink (UL) are subject to fast inner loop power control. The power control loops steer the powers of the UEs, so as to achieve the Signal-to-Interference-and-Noise Ratio (SINR) targets set for the connection of the UEs. This in turn means that all power control loops are coupled nonlinearly, via the SINR measurement. It is well established in the literature that the loops stay stable as long as the UL is operating below the pole capacity of the cell. However, when the system is close to the pole capacity stability is reduced and so called power rushes can occur because one or several of the users can not reach their SINR targets. This is a very significant problem in WCDMA since the power control loops are very fast and capable of stepping up the UE power with 1500 dBs/second. Means to prevent such power rushes are therefore a necessity in case the UL of the WCDMA system is to be operated close to the pole capacity.
In near future versions of WCDMA, Interference Suppression (IS) and time division scheduling (TD) will be used in order to achieve better performance in terms of e.g. peak data rates, coverage, system throughput and system capacity. However, the power rush problems described above become even more pronounced with TD and with IS. TD schedules High Data Rate (HDR) users per Hybrid Automatic Repeat-reQuest (HARQ) per 2 ms Transmission Time Interval (TTI), which corresponds to 3 time slots (with 1500 slots/second, or 0.667 ms per slot). This may give rise to very sudden and relatively large changes in the load. IS often allows and requires more aggressive scheduling in order to reach the full performance potential. Since IS gains sometimes disappear because of inter-antenna branch correlation, aggressive scheduling can make the WCDMA uplink more prone to power rushes. The SINR measures will increase considerably by a disappearance of the IS gain, initiating an intense power control activity.
Fast congestion control (FCC) is a set of methods to prevent power runaway in the UL. FCC may use various measurements to achieve its goal, however a high BandWidth (BW) measurement of, or associated with, the WCDMA UL load is a necessary ingredient. Here, high BW means that the measurement is made as fast as the sampling of the fast power control loop, i.e. in WCDMA at a rate of 0.667 ms. The delay of this measurement, as compared to the power control loops must not be more than 1-2 times 0.667 ms.
One problem is thus that such fast measurements with low delay are not available in systems with old HardWare (HW). Such HW does not generally support TD or IS and has typically a limited memory capacity. Hence, there is no option to easily introduce high BW or low delay measurements. Within existing solutions, there is no known way to perform FCC in WCDMA systems when the available RoT measurement has a significantly lower bandwidth than the fast inner loop power control loops of the WCDMA UL. Any low cost congestion control solution therefore needs to rely on existing measurements. Also, in near future systems with new HW, TD and/or IS will probably be introduced before fast congestion control is introduced. Any congestion control introduced before FCC therefore needs to rely on existing measurements.
A more specific problem that has been found to arise without a well operating FCC and that have no prior art solution is a high risk that the powers of the UEs may increase significantly. In worst case a power increase of 15-20 dB can take place before any action at all can be taken. By breaking the power control loop, such a rush can indeed be avoided. However, this action has further consequences in that there is a high risk that UEs may re-initiate a power rush, unless power control loops are kept broken until the next available RoT measurement appears. Furthermore, there is a high risk that e.g. fading effects may reduce the SINR of the users to unacceptably low levels. Such low levels may possibly cause dropped calls in case power control loops are long. This can be the case, e.g. if power control loops are kept broken until a next RoT measurement arrives.
The overall consequences for the prior art situation include that systems with old HW will not be able to use a more aggressive scheduling. Thereby, little performance improvement will be possible even when other improvements support more aggressive scheduling. Furthermore, systems with new HW and TD and/or IS will not be able to utilize the full performance benefit associated with these features, that is associated with aggressive scheduling.